1. Field of the Invention
The present invention relates generally to a gas sensor which uses a solid electrolyte for determining the concentration of a component in a gaseous fluid, and a method of manufacturing the same, and more particularly to such a gas sensor which is capable of stably providing a characteristic curve for high sensing accuracy, and a method for manufacturing the same.
2. Discussion of the Prior Art
There has been known a device which incorporates an electrochemical cell using a solid electrolyte. For example, such an electrochemical device is used as an oxygen sensor having an electrochemical cell which consists of an oxygen-ion conductive solid electrolyte such as zirconia ceramics, and a pair of porous electrodes, for determining the concentration of oxygen in an exhaust gas produced by an internal combustion engine of a motor vehicle. In this type of sensor, an electrochemical pumping action is performed based on the reaction of the electrodes which occurs while an electric current is applied between the electrodes. In the meantime, one of the porous electrodes is held in communication with a measurement gas in an external space, via suitable diffusion resistance means such as a pin hole, a thin flat space or a porous ceramic layer, which provides a predetermined diffusion resistance to a flow of the measurement gas. The sensor provides an output in the form of a pumping current which corresponds to the oxygen concentration of the external measurement gas. Also known are electrochemical devices or gas sensors or detectors adapted to detect hydrogen, carbon dioxides, fuel gases, etc., by utilizing the principle based on the electrochemical pumping action and the diffusion resistance, as practiced in the oxygen sensor indicated above.
In a gas sensor using such an electrochemical cell (pumping cell) capable of performing an electrochemical pumping action, the measurement gas undergoes bulk diffusion through the diffusion-resistance means if this means consists of a pin hole or a thin flat space, or alternatively undergoes Knudsen diffusion if the diffusion-resistance means consists of a porous ceramic layer. In the former case, the gas sensor considerably suffers from a problem that a limit current obtained by the cell depends largely on the temperature of the sensor. In the latter case, the temperature dependency of the limit current tends to be a negative value, and the diffusion resistance of the porous ceramic layer depends largely on the pressure of the measurement gas. Further, the sensor using the porous ceramic layer tends to have a local variation in the diffusion resistance, due to a local variation in the porosity or size of the pores (voids) or presence of local cracks, whereby the distribution of concentration of a desired component of the measurement gas adjacent to the inner electrode is varied from one location on the electrode to another. Thus, the characteristic curve (pumping current-pumping voltage curve) to be obtained tends to be dull, leading to insufficient sensing accuracy of the gas sensor.
There is also known a gas sensor which utilizes a porous ceramic layer whose pore diameter is adjusted so that the measurement gas undergoes both the molecular diffusion and the Knudsen diffusion through the porous diffusion-resistance structure, whereby the temperature dependency of the limit current is reduced. However, the adjustment of the diameter of the pores of the porous structure may cause an undesirable change in the diffusion resistance of the porous structure, and consequently a considerable variation in the limit current obtained.
It is considered possible to construct the gas sensor such that one of the electrodes of the electrochemical pumping cell is exposed to a comparatively large internal space or cavity formed within the interior of the cell, while the internal space is held in communication with the external measurement-gas space through the diffusion-resistance means in the form of a pin hole filled with a porous body. In such a gas sensor, however, the internal space may be a useless or undesirable space. Namely, it takes a longer time for the atmosphere within the internal space to be changed according to a change in the measurement gas in the external measurement-gas space, and the operating response of the sensor is thus deteriorated. Another problem of the gas sensor of the type indicated above lies in that the concentration of the measurement gas within the internal space is fluctuated due to flows of the gas when the pressure of the measurement gas in the external space is periodically changed. In this instance, the sensing accuracy of the sensor is lowered.
Laid-open Publication Nos. 58-19554 and 60-13256 (published in 1983 and 1985, respectively) of Japanese Patent Applications disclose another type of gas sensor, in which an air gap formed around one of the two electrodes of the electrochemical pumping cell communicates with the external measurement-gas space, via a porous diffusion layer or a porous sputtered ceramic layer. Since the porous layer is formed face to face with the electrode, the thickness of the air gap may be reduced between the porous layer and the electrode, which may cause an innegligible diffusion resistance and a consequent gradient in the concentration of the introduced measurement gas on the electrode, thereby preventing the sensor from obtaining a sharp characteristic curve for accurate measurement of the concentration. If the case where the presence of the porous layer does not significantly reduce the thickness of the air gap, or the diffusion resistance provided by the gap is negligible, the air gap may be an unnecessary space that deteriorates the operating response of the sensor.
Laid-open Publication No. 59-163558 (published in 1984) of Japanese Patent Application discloses a gas sensor in which a thin flat space to which one of the electrodes of the electrochemical pumping cell is exposed communicates with the external measurement-gas space, through a porous body which defines an end of the thin flat space, so that the measurement gas diffuses through the porous body into the thin flat space. This arrangement suffers from a low limit current and a low S/N ratio. In addition, since the cross section of the porous body part which forms an inlet of the thin flat space is relatively small, the atmosphere within the thin flat space is easily affected by a local plugging of the porous body, and the porous body is difficult to be formed with consistent porosity.